29er sizing and gearsets

[balap]

After riding a road bike for few years I feel like getting a 29er for occasional commuting to work and possible off road rides.
I planning to get a second hand bike, maybe something like Giant talon, xtc etc. The brand is not important.

I am 1.7 m (5"7) and is between size S and M according to most bike websites.
In my case, is it better to go for size S or M?

My budget is up to $800 (for a used bike) so will probably get me alivio or deore gearsets.
Are they big differences between the two?
I am most likely not going to use the bike off road much.

Thanks in advance.

[Duck!]

Try both sizes & see which one you prefer. Common wisdom for mountain bikes is if you're borderline on size, go the smaller one, as it's more lively handling, and gives more clearance around "sensitive bits" when chucking it through rough & twisty stuff. If you're not off-roading much this is less of a concern, however standover clearance, especially considering the inherent height increase due to big wheels & fat tyres may still sway your choice.

As far as gearing goes,the biggest difference is in that Deore is 10-sp, while Alivio is 9-sp. However, contrary to expectation, Alivio will actually give a wider spread of gears, due mainly to the 22-32-44T chainring combination vs the 24-32-42 of the 10-sp. Deore. The 22/34 low gear of Alivio is pretty well lineball with Deore's 24/36, but gains a bit more at the top due to the bigger big ring (both cassettes have 11T at the top). The 10-sp. is smoother through the mid-range due to slightly tighter ratio jumps.

[balap]

Thank you for the advise.

I have to overcome feeling guilty going to a shop trying bikes knowing that I won't buy it from them.
My road bike Trek Madone is size 54 cm. It fits me well.
I like the idea of cross country on easy dirt/gavel roads. Nothing too adventures.

It sounds like Alivio or Deore will be alright for me.